What's the best tool for booking flights and why?

Can't recommend Skyscanner enough. Cheapest deals, checks multiple dates easily, and a clean UI.

Agree with Nick on this one. One highlight is that it's great for flexible travel and discovery, i.e. you want to find the cheapest time to go somewhere or you want options of places to go at a certain time.

I like Skyscanner because of the Explore button. You can select a month, countries of destination (e.g. Netherlands -> Anywhere ) and find a greatest deals for this month to go. Moreover you can select one way ticket and catch the best deal to e.g. Rome from Netherlands, but return ticket buy from Florence to Nethrlands and between Rome-Florence travel by car or train.

Google Flights has gotten quite good — it allows me to track flights over time so I can get the best price at the right moment. It's not as full featured as some other tools (and it's kind of cramped in Google's search UI) but it's pretty solid all around.

I've been using Hipmunk for years. I don't like flying. So I generally care the most about time/airline/layovers. I like their "agony" rating. 😀 I generally book as soon as I know when I need to go somewhere. And will adjust my trip days if there is a large drop in price ($100 difference between a Monday or Tuesday for example). Hipmunk makes checking multiple dates easy.

Because of Google's cramped UI, I still prefer Hipmunk. Their timeline view is so clear and helps compare options so well.

My information search starts here. They claim this - "We're better at finding cheap flights than anyone else— so good, United Airlines actually sued us for it". They show hidden city ticketing trips in addition to what sites like Expedia, KAYAK, and Travelocity show. Taking a stop might save hundreds of dollar as @melaniepinola mentioned here - http://lifehacker.com/skiplagged... You can also track previous searches for changes in price. Available on iOS/Android.

Hitlist is less useful when you're not flexible, but when you have a destination in mind but don't really have a specific travel date to aim for, this is what you need.

Disclosure: I'm the founder. As Chris says, Hitlist isn't for everything - if you know exactly where and when you're traveling, we won't offer a better experience than setting up a Google flight alert. But if you are trying to figure out the cheapest place to travel in a time window or the cheapest time to fly to a place, Hitlist offers a convenient and fun way to see what's available and track your favorite destinations. Use it to know when to fly to Japan, or find the best kitesurfing destination to visit the second week in August.

I love their HackerFare concept of finding a cost effective way to reach a destination even if it involves bookings from different airways. Their fare trend graph also provides you with forecast of the fare, and best time to book them.